The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Optical recording devices are used to record information, such as music, movies, pictures, data, etc., on recordable media. Examples of recordable media are compact discs (CDs), digital versatile discs (DVDs), high density/high definition DVDs and Blu-ray Discs (BDs). In order to record information onto recordable media, a recording device typically tracks the location of a laser beam on the recordable media.
FIGS. 1A-1B respectively illustrate partial cross-sectional views of examples of recordable mediums 10-A, 10-B. The recordable mediums 10-A, 10-B have lands 12 and grooves 14, which are formed on one or more recording layers 16 of one or more main substrates 18. The main substrates 18 may be adhered via an adhesive layer 20 to dummy substrates 22, as shown.
The lands 12 and grooves 14 refer to physical structures of the recording layer 16 that are adjacent each other but that have different associated depths. For example, the grooves 14 have a greater associated depth than the lands 12. Sample land depths D1 and sample groove depths D2 are shown. The depths may be measured relative to a disk outer surface 24 and are equal to a fraction of an optical wavelength of a laser beam. The lands 12 and grooves 14 provide servo information for positioning of a laser beam spot on a disc. The lands 12 and grooves 14 also provide reflected beam signal modulation that is detected and used for tracking.
Standards, such as DVD+/−R and DVD+/−RW, require recording only over grooves. An alternative standard, referred to as DVD-RAM, requires recording over both land and groove structures. In DVD+/−R and DVD+/−RW recording, the lands 12 and grooves 14 typically form a continuous spiral. In DVD-RAM recording, the lands 12 alternate with the grooves 14 to form a continuous spiral.
FIG. 2 illustrates a conventional optical DVD drive system 50. The optical DVD drive system 50 includes a laser source 52, such as a laser diode, that emits a laser beam 54. The laser source 52 may be part of an optical read/write assembly (ORW) 56. The ORW 56 includes a collimator lens 58, a polarizing beam splitter 60, a quarter wave plate 62, and an objective lens 64. The laser beam 54 is collimated by the collimator lens 58 and passed through the polarizing beam splitter 60. The laser beam 54 is received by the quarter wave plate 62 from the beam splitter 60 and is focused via the objective lens 64. The laser beam 54 may be radially displaced across tracks of the optical storage medium 68 through movement of the ORW 56 via a sled motor 66. The laser beam 54 is moved while the optical storage medium 68 is rotated about a spindle axis 69. The laser beam 54 is shaped and focused to form a spot over the land/groove structures of an optical storage medium 68 via lens actuators 70.
The light from the laser beam 54 reflects off the optical storage medium 68 and is thus directed back into the ORW 56. The reflected light, represented by dashed line 72, is redirected by the beam splitter 60. An astigmatic focus lens 76 focuses the reflected light into a spot over a photo-detector integrated circuit (PDIC) 74. Although not shown, additional photo-detectors may be used to detect other diffracted light beams, which are also not shown.
Referring now to FIG. 3, an exemplary bit-stream of write channel data is presented. Data that is to be written to optical storage media may first be encoded using techniques such as cyclic redundancy check (CRC), error-correcting code (ECC), Reed-Solomon coding, and/or interleaving. Alternatively, 8-to-14 modulation (EFM) may be used to encode the data to be stored on an optical storage medium. The encoded data stream may then be sent to a laser driver unit, which converts the data stream into a series of electrical pulses that are used to record the data onto the optical storage medium.
An exemplary channel bit-stream is represented as waveform 80. The waveform 80 contains one bit for every time period (T). The interval where the waveform 80 is high may be referred to as a space 82a, 82b. The intervals where the waveform 80 is low may be referred to as marks 84a-84c. Marks may be represented on the optical storage media as areas of low reflectivity (pits), amorphous domains, or any other type of form that can be sensed by the optical system. Spaces may be represented as areas of high reflectivity between marks. These reflectivities may be created by a laser beam, as is known in the art.
A typical optical reader, for example a DVD player, has a light spot that is approximately 9T wide. In other words, a typical optical reader detects the reflectivity of an area on the optical storage medium that is nine time periods long. Thus, marks or spaces of a length less than 9T may be difficult to distinguish from adjacent spaces or marks. In most encoding schemes, optical readers are generally designed to detect edges of a waveform (e.g., edges 85 of waveform 80) in order to decode the data therein.
Depending on the parameters of the optical storage media, and the binary encoding scheme employed, the length of marks and spaces may be constrained. For example, in the EFM encoding technique, the smallest length of a mark or space is 3T and the longest length of a mark or space is 11T. A laser driver, based on the information in the data stream, determines the correct power and time duration for operating a laser to create marks (e.g., marks 84a-84c). These marks 84a-84c, in combination with spaces 82a, 82b, are then detected or read by an optical reader. The goal of the laser driver is to create marks and spaces such that the optical reader will detect the stored data correctly. In accordance with this goal, a laser driver typically includes a translation module, such as a laser table, that dictates how data is to be stored on an optical storage medium.
Among other functions, a laser table will direct the laser driver to position a mark based on the immediately preceding space length and the length of the mark to be written. Thus, the actual physical beginning of a mark edge 85 may need to be adjusted from a desired position (that is, the position corresponding substantially to the intended data stream) in order for the position that is detected to correspond with the desired position of the signal to be stored. In other words, the optical reader will detect an edge 85 in a position that may not correspond to the actual physical position of the edge as written on the optical storage medium. This phenomenon is referred to as inter-symbol interference or ISI and causes the waveform that is output from the optical reader to differ from the physical marks and spaces that are stored on the optical storage medium. ISI contributes to jitter and other causes of error in data retrieval.